A polythiol compound and a polyisocyanate compound have been commonly used as raw materials of a urethane optical member. An aromatic polyisocyanate compound in the polyisocyanate compound is being produced in large quantities inexpensively and may contribute to enhancement of the refractive index, and thus an aromatic polyisocyanate is preferred as a raw material of a urethane optical member which particularly requires a high refractive index. In particular, 4,4′-diphenylmethane diisocyanate (which may be abbreviated as MDI), and 2,4-tolylene diisocyanate and 2,6-tolylene diisocyanate (which may be abbreviated as TDI) are useful as a raw material monomer of an optical member having a high refractive index from the standpoint that these compounds are easily available industrially and facilitate enhancement of the refractive index.
For example, PTL 1 describes the use of a thiol compound having a particular structure with an aromatic polyisocyanate compound as raw materials for a polymerizable composition used for an optical resin capable of achieving a high refractive index, and 2,4-tolylene diisocyanate and 4,4′-diphenylmethane diisocyanate are used in the examples and comparative examples thereof.